Powders composed of coarse and fine particles are utilized in many industrial processes. Examples of powders include foods, pharmaceuticals, abrasives, pigments, plastics, magnetic coating materials and the like. The particles may range in diameter from less than 1 micrometer to 1000 micrometers or more. In the industrial processing of powders, it is useful to characterize the physical properties of the powders being processed. Important powder properties include particle size distribution, particle cohesiveness and powder flowability. A particle sizing system for particle size measurement utilizing a time-of-flight technique is disclosed in U.S. Pat. No. 4,895,034, issued Jan. 23, 1990 to Poole. A system for dispersing dry particles in a gas stream for measurement is disclosed in U.S. application Ser. No. 08/204,476, filed Mar. 1, 1994 and assigned to the assignee of the subject application.
Powder flowability is an important characteristic in processes which involve the transporting of powders. Examples of such processes include flow of powders into molds, flow of powders through pneumatic systems and flow of powders to and from containers, such as trucks. The flowability may affect the energy required to transport the powder. Furthermore, powders which have poor flowability characteristics may cause blockages in powder transport systems. Powder flowability depends on powder type, particle size, moisture content and the presence of impurities. A powder with high moisture content may have very different flowability from the same powder with low moisture content.
One prior art technique for determining flowability involves passing the powder being characterized through a device similar to an hourglass. The powder passing through the device forms a peak having an angle of repose with respect to horizontal. The angle of repose is the maximum angle which may be present in a powder sample and is a measure of powder flowability.
A system for characterizing powder avalanche in a rotating drum is disclosed by B. H. Kaye in Powder and Bulk Engineering, February, 1996. In the disclosed system, a light beam is directed through a transparent, rotating drum containing a powder sample. As the drum rotates, the powder avalanches at periodic intervals. A photocell array positioned on the opposite side of the drum is blocked to a greater or lesser degree as the powder avalanches within the drum. The output of the photocell array represents powder avalanching within the drum. While the disclosed system provides generally satisfactory results, particles of the powder sample may stick to the walls of the sample drum, thus giving an erroneous indication of avalanching.